The field of the invention is that of the transmission of digital data by RF channels. More specifically, the invention relates to the transmission of information dedicated to a particular receiver, or to a group of identified receivers.
The invention can be applied especially to systems of digital transmission in which at least some of the receivers have an intermittent operation, i.e. they go into operation periodically, at predefined instants, so as to economize energy.
These may be, for example, receivers capable of receiving particular information (alarms, road information, time schedules for transportation means, magazines, medical services) or personal information (paging, faxes, calendaring information).
A particular field of application of the invention is that of one-way paging. Paging is understood to mean any system enabling the transmission of a message to a portable receiver that can advantageously be kept in a user's pocket. These systems can be used in particular to transmit a message to a person who is travelling when the sender of the message does not even know where his or her correspondent is.
Conventionally, such systems in the most simplest case enable the transmission of a sound and/or visual signal by a pocket receiver upon reception of a message. These systems are generally known as simple private call systems.
Other more elaborate receivers enable the reception and display of a short digital message such as for example a telephone number to be called or a coded piece of information.
Receivers of this kind are used especially in paging systems known as the "Operator" (registered mark) and "Alphapage" (registered mark) systems available in France.
These paging systems, as well as all other presently known paging systems, have many drawbacks.
First of all, they can be used at best to transmit only a short message. Consequently, in practice the subscriber receiving a message must always look for a telephone to find out the actual contents of the information intended for him or her.
Furthermore since, for obvious reasons of operating autonomy, portable receivers can be supplied only by batteries or cells, these receivers must have intermittent operation. More specifically, a paging receiver is most usually on standby (only a clock is powered) and goes into operation cyclically to detect a message, if any, that is intended for him or her.
This implies a particular management of the instant at which transmission is made so that each message is sent only when the destination receiver has left its standby state to go into operation.
By contrast, it is necessary that the receivers should have specific synchronization means so as to control the instants at which they go into operation. To do this, in known systems, temporal synchronization data elements are inserted among the paging data elements.
The corresponding synchronization operations hold the receiver in operation for a certain period of time leading to high energy consumption.
Furthermore, known broadcasting systems encounter difficulties in the reception of FM signals in many situations especially when they are mobile and/or they are used in urban environments (because of the Doppler effect, fading, multiple path echoes, scrambling, etc.). They therefore do not provide sufficient guarantee of reception.
In order to overcome some of these drawbacks, the patent application FR 9204479 entitled "Procede de transmission de donnees numeriques de radiomessagerie, et recepteur de radiomessagerie correspondent" (Method for the transmission of paging digital data elements and corresponding paging receiver) filed by the same Applicants as the present application, proposes a system of paging integrated into a more general system of digital broadcasting implementing time and frequency interlacing techniques such as the COFDM.
The use of this system can be used to provide for the transmission of data in an efficient way, even when the transmission channel is highly scrambled. Furthermore, no new infrastructure is required since the same medium is used as for broadcasting (DAB standard).
In this known system, there is provided a fast information channel (FIC) including flags to indicate the presence of information corresponding to the services given. To limit the energy consumption, this FIC channel is not frequency-interlaced. The paging data elements may be inserted into this FIC channel if they are small in number. Otherwise, they are placed in a main service channel (MSC). In this case, they undergo temporal and frequency interlacing.
In practice, it is noted that the place available in the FIC channel is insufficient to enable quality services, especially the supply of information of great length.
This is especially the case for the new applications of broadcasting data to portable terminals (known as PDAs or personal data agents) used for example for services for the transmission of files, road information, general information, stock exchange information, fleet management information, etc.
It is therefore necessary to place at least a part of the paging data elements (and more generally of identified data elements, namely data elements that can be identified by one or more destination receivers) in the MSC. This raises a major drawback, namely the storage of several frames (16 in the case of the present DAB standard) to carry out the de-interlacing and therefore to reconstruct the messages.
Furthermore, the decoding of a useful data element requires the taking into account of a large number of unnecessary data elements (especially all the data elements contained in the FIC and in the channel in which the useful information is contained).
This technique is therefore clearly incompatible with an optimization of reception in receivers with limited autonomy which must have an operation that is intermittent and limited to the maximum extent in time.